Abstract
A recent publication, (Easthope in IMA J. Appl. Math. 80(2):554–568, 2015), determined that the Simplified General Perturbations 4 (SGP4) expression for the rate of change of the mean anomaly M is missing a secular term of order J 2. It was also speculated that the adverse effects of this missing term on propagated satellite trajectories are being mitigated by means of the B ∗ drag parameter provided in the Two-Line Element Set (TLE). By making use of satellite truth trajectories combined with analysis, this paper shows that, in fact, drag is not being used in this manner, but that the TLE value of the mean motion, n 0, has been modified to provide the necessary correction and enable acceptably accurate SGP4 propagation.
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